Segmented message traffic occurs in communications networks whenever traffic messages are divided into shorter length message segments or segmented message units. For instance, traffic messages in the form of variable-length packets such as Internet Protocol (“IP”) packets or Frame Relay (“FR”) frames are oftentimes segmented into fixed-length message segments such as Asynchronous Transfer Mode (“ATM”) cells. This may take place, by way of example, in network architectures for which traffic conforming to variable-length message protocols is transported over a network backbone or other trunking network space that employs a fixed-length message protocol. As used and understood herein, message traffic is broadly intended to comprise all forms of network traffic such as cells, packets, frames or other units of transmission, whatever the format, function or content thereof.
Where the transmission of segmented message traffic is subject to traffic congestion, various known techniques of message discard have developed for assisting with congestion recovery. For example, traffic management standards as known to those in this art have specified numerous methods of intelligent message discard for segmented traffic. One such example is to discard segmented traffic upon the occurrence of congestion at the level of the non-segmented messages rather than at that of the constituent segmented message units, as set out in the ATM Forum Traffic Management Specification, Version 4.0, document number af-tm-0056.00, published by the ATM Forum in April 1996 (hereafter the “ATM Traffic Management Specification”). Another example of same is as set out in Recommendation I.371, Traffic Control and Congestion Control in B-ISDN, published by the International Telecommunications Union-Telecommunications (ITU-T) in March 1993 (hereafter the “ITU-T B-ISDN Traffic Control Specification”). In the case of FR traffic over ATM, the reaction to congestion according to this approach is to discard traffic at the frame level rather than at the cell level.
More specifically, it has been known in the art of communications networks to apply certain techniques of early message discard to segmented message traffic as a means of achieving traffic management. Thus, a network device that operates according to a technique of early message discard will, when in a congested state, reserve its buffer capacity for segmented message units which form part of segmented messages already admitted therein. Any orphaned segmented message units forming part of an incomplete message will therefore not be transmitted downstream of the congestion. Thus, the object of early message discard is to reject complete non-segmented messages once congestion is encountered. This precludes such orphaned segmented message units from unnecessarily utilizing downstream network resources, thereby maintaining good network throughput. An example of this type of early message discard known to those in this art is the Early Packet Discard (“EPD”) technique as defined in the ATM Traffic Management Specification. The known EPD technique operates at the level of Protocol Data Units (“PDUs”) pursuant to ATM Adaptation Layer 5 (“AAL5”), all as defined in Recommendation 1.363, B-ISDN ATM Adaptation Layer (AAL) Specification, published by the International Telecommunications Union-Telecommunications (ITU-T) in March 1993 (hereafter the “ITU-T ATM AAL Specification”).
Although it is known to configure EPD techniques in respect of a given discrete traffic stream such as at the level of a virtual connection (“VC”), in many network topologies such discrete traffic streams are associated with an aggregated traffic stream of which they form a constituent part. For instance, an aggregated traffic stream of the foregoing kind may be configured as a virtual path (“VP”). In these circumstances, it would be desirable to enable message discard techniques at the level of a frame or packet on the constituent VCs of each VP. However, known techniques of message discard that are applicable to VP traffic do not typically discriminate as to the constituent VC traffic forming part of the aggregated VP traffic. This is problematic in that cells or frames may improperly be delineated and discarded indiscriminately within the aggregated VP traffic stream by operation of such prior art message discard techniques, thereby causing AAL5 frame corruption within one or more constituent VC traffic streams.
It has been known in the art to implement Early Packet Discard techniques at a per-VC level by individually configuring Early Packet Discard capability for each VC of a VP. However, this known practice is associated with the disadvantage of requiring the implementation of an Early Packet Discard algorithm typically for each and every constituent VC of a VP, which can number in the tens of thousands or more. There is therefore a need for an apparatus and method that would achieve early message discard for segmented message traffic in an aggregated manner while retaining the ability to discriminate as to constituent message traffic streams.